This invention relates to an organic composite-plated steel sheet applied with a film coating and more particularly it relates to an organic composite-plated steel sheet easily applied with a film coating which is superior in press workability, spot weldability and electrodeposition property and especially high corrosion resistance.
Recently, demands for reduction of cost and improvement of corrosion resistance of zinc or zinc base alloy plated steel sheets for automobiles and household appliances have increased and research on new products has been widely made.
For example, weldable steel sheets comprising a basic steel sheet film-coated with an electrically conductive coating composition containing an electrically conductive metal powder are disclosed in Japanese Patent Kokai No. 61-23766 and Japanese patent Kokoku No. 62-20024. As a representative thereof, Zincrometal has been offered as markets. However, these suffer from practical problems such as dragging caused by press working and flaws formed by sticking of peeled film to mold, resulting in damages of appearance of a pressed product.
Furthermore, improvement in corrosion resistance by application of an aqueous solution of a mixture of water-dispersible emulsion resin with a highly corrosion resistance chromic acid or a chromium compound onto the surface of zinc or zinc base alloy plated steel sheet are proposed in Japanese Patent Kokoku No. 55-51032 and Japanese Patent Kokai Nos. 59-162278 and 61-584. Although high in corrosion resistance, these sheets also have practical problems such as sweating and dissolving-out or chromium with aqueous treating solutions. Moreover, coating of a blend of water-dispersible emulsion resin and an organic composite silicate (silica sol, silane coupling agent) is proposed in Japanese Patent Kokai Nos. 60-50181 and 60-149786. According to these methods the problem caused by dissolving-out of chromium has been considerably improved, but since silica is contained as a stable colloid, alkali ion or ammonium ion is present which also causes problems in the properties of the coating film, especially water resistance. Further, in case of solvent type coating compositions containing powdered silica, the viscosity of the coating composition increases very much due to its structural viscosity and coating in uniform film becomes impossible. The viscosity may be reduced by breaking the structural viscosity with hydrogen bond of alcohols, but this method cannot be applied to the system where polyisocyanate compounds are used as curing agent.
Thus, thin film-coated corrosion resistance steel sheets have been required to have (1) higher corrosion resistance, (2) higher adhesion to coating film (especially cation electrodeposited film) formed on the steel sheet and (3) lower baking temperature (140.degree.-170.degree. C.) for improvement in working strength of the steel sheet. Various coating compositions have been proposed which can be expected to afford desirable results. However, since there is the problem of thixotropy in the coating composition as mentioned above which is considered to be especially significant in the system where polyisocyanate compound is used as a curing agent, none of practically applicable coating compositions containing organic solvent-soluble epoxy resin, polyisocyanate compound and silica particle have been obtained as those for solvent type thin film-coated corrosion resistance steel sheets.